1. Field of the Invention
The present invention relates to a device structure using a thin-film material and also to a method of fabricating such a device structure. More particularly, the invention relates to a device structure applied to a sensor for measuring the flow rate of a fluid and also to a method of fabricating this device structure.
2. Description of the Related Art
Instruments for measuring flow rates are generally known as flowmeters or fluid meters. One known type of such instruments makes use of a thermistor. In particular, this known instrument utilizes the fact that when heat is carried off by a fluid, the temperature of the thermistor drops. Generally, if the thermistor is in contact with the fluid, the amount of heat carried away from the thermistor depends on the flow rate or flow velocity. Therefore, a certain correlation exists between the output from the thermistor and the flow rate. Hence, the flow rate can be calculated from the output from the thermistor.
The flow rate is the product of the cross-sectional area of the fluid and the flow velocity. It is now assumed that a fluid is flowing through a circular pipe having an inside diameter of r at a flow velocity of v. The flow rate is given by v.pi.r.sup.2, for example in liters/min. Therefore, if the cross-sectional area of the fluid is known, the flow rate and the flow velocity can be found simultaneously.
Generally, a thermistor refers to a semiconductor having a large negative temperature coefficient. Intrinsically, however, a thermistor means a thermally sensitive resistor and so thermistors should not be restricted by the signs (positive or negative) of their temperature coefficients or by their materials. Consequently, metals such as platinum having positive temperature coefficients can be referred to as thermistors.
Devices using materials whose resistances are changed by temperature variations such as thermistors are collectively known as temperature measuring resistors, temperature sensing elements, or resistance thermometers. Also, it can be said that materials whose resistances are varied by temperature variations perform thermistor functions. Materials whose resistances are varied by temperature variations are hereinafter referred to as temperature measuring resistor.
The above-described method of measuring flow rates employs a thermistor. In another method, a resistive heating element which produces heat by the Joule effect is exposed to a fluid. The amount of heat carried away from this heating element depends on the flow rate. The electrical current flowing through the heating element is measured. In this way, the flow rate can be calculated.
In a further method, a fluid is made to carry away heat from a heating element in contact with the fluid. The amount of heat carried by the fluid is measured by a separate temperature measuring resistor such as a platinum sensor. In this way, the flow rate is computed.
These known flowmeters fabricated by the prior art techniques have the problem that their measurable range is limited. For example, the range is given by only one or two digits.
We have fabricated a flowsensor using a thin diamond film having a high thermal conductivity. Flow rates were measured, using nitrogen gas. This diamond flowsensor was composed of the thin diamond film on which a heating element consisting of a thin platinum film and a temperature measuring resistor were installed. The diamond film is several micrometers thick and several millimeters square.
During measurement, the thin diamond film was maintained in a thermally floating state as accurately as possible. The diamond film was brought into contact with a fluid. Heat in the form of pulses was applied to the diamond film from a heating element. The resulting changes in the temperature of the diamond film were measured by a temperature measuring resistor.
At this time, the thin diamond film was quickly heated by the heat in the form of pulses and then cooled. Correspondingly, the output from the temperature measuring resistor varied quickly and regained the original value. That is, the temperature measuring resistor produced a certain responsive waveform. Since the area of this waveform accurately corresponded to the flow rate, the area of the responsive waveform produced from the temperature measuring resistor was calculated whenever heat in the form of a single pulse is applied. In this way, measurement of the flow rate could be performed.
More specifically, heat in the form of a pulse persisting for 0.18 second was applied every 4 seconds. The area of the responsive waveform was calculated. In this way, the flow rate was measured every 4 seconds. In practice, flow rates could be measured over a wide range from tens of sccm to hundreds of thousands of sccm. It can be said that this method of measurement produces a much wider dynamic range than the prior art flow rate-measuring instruments such as mass flowmeters.
In the above-described structure, the flow rate is calculated from the characteristics of the response of the thin diamond film to the applied heat in the form of pulses. It can be understood that the response characteristics of the diamond film reflect the thermal effect of the surroundings on the film.
The principle of the above-described measurement of flow rates can be understood in the manner described now. When a fluid touches the thin diamond film, an amount of heat corresponding to the flow rate of the fluid is carried away from the diamond film. In other words, the diamond film is thermally affected by the fluid by an amount corresponding to the flow rate of the fluid. This thermal effect is reflected in the characteristics of thermal response to the applied heat in the form of pulses. This response characteristics affect the manner in which the diamond film is heated by the heat in the form of pulses and the manner in which the film is cooled and thus can be observed.
These response characteristics reflect thermal effect of the surrounding on the diamond film. Also, the thermal effect corresponds to the flow rate of the fluid flowing in contact with the diamond film. As a result, the response characteristics correspond to the flow rate of the fluid flowing in contact with the diamond film.
The foregoing discussion is based on the ideal assumption that the thermal effect on the diamond film depends on the flow rate of the fluid flowing in contact with the diamond film. However, where a thin-film material is contacted with a fluid and the flow rate is measured, the thermal effect on the thin-film material can be attributed to the following factors:
(1) Thermal action between the thin-film material and the fluid.
(2) The amount of heat flowing out of the thin-film material into a base that holds the thin-film material.
(3) The amount of heat carried away from the thin-film material via lead wires or conductors.
Of these factors, a flowsensor needs only the thermal action (1) above. The thermal effects due to (2) and (3) should be minimized. Therefore, it is necessary to realize a structure which does not permit heat to be carried away from the thin-film material except into the fluid. That is, it is important that the thin-film material be thermally insulated and retained so that the thermal effect on the thin-film material arises mainly from the fluid flowing in contact with the thin film.
It is quite useful to use a thin diamond film as the thin-film material. However, it is difficult to machine a thin diamond film into a desired device shape. For example, it is difficult to even pick up a thin diamond film having a thickness of less than 5 .mu.m with forceps.
A known method of machining a thin diamond film uses irradiation of laser light. For example, cutting of CVD thin diamond films, using YAG laser light, is reported in "New Diamond, Vol. 6, No. 2, p. 36" and in "The proceedings of the Society of Accurate Machining of Japan, 56/12/1990".
Where a thin diamond film is formed by CVD on a silicon wafer and then it is machined by irradiation of YAG laser light, the diamond film peels off, breaks, or cracks, because a stress is exerted between the silicon wafer and the diamond film.
Where a device using a thin diamond film should be fabricated, it is necessary to fabricate an electronic apparatus or circuit on the surface of the diamond film. However, this fabrication is not easy to carry out, because the diamond film is in a mechanically unstable state. That is, it is difficult to fabricate an electronic apparatus or circuit on the surface of a thin diamond film which is formed on a silicon wafer.
Indeed this problem may be alleviated by selecting a substrate other than silicon wafer or by appropriately selecting conditions in which the diamond film is fabricated, but the problem cannot be essentially solved. A method of increasing the strength of the film itself by increasing the thickness above 100 .mu.m may be contemplated. However, increasing the thickness is not desirable from an economical point of view.